In manufacturing formed bodies such as can bodies or can lids used as beverage cans or the like, an aluminum sheet or an aluminum alloy sheet is used. Although aluminum exhibits excellent formability and excellent flavor keeping property as a can body material and can lid material, aluminum has a drawback that aluminum exhibits less corrosion resistance compared to a surface-treated steel material.
On the other hand, a chromate-based chemical conversion treatment agent has been used for applying surface treatment to the aluminum sheet. A chromium-based chemical conversion coating film formed on the aluminum sheet by the chromium-based chemical conversion treatment agent exhibits the excellent corrosion resistance even when used alone and also exhibits excellent corrosion resistance and adhesiveness even after various resin-based coating materials are applied to the chromium-based chemical conversion coating film. Accordingly, the chromium-based chemical conversion coating film has been used in broad applications including architectural materials, electrical appliance for household use, fin materials, vehicle-use evaporators and beverage can materials (see patent document 1).
However, the chromium-based chemical conversion treatment agent uses chromium metal which is harmful to humans in a surface treatment liquid and hence, recently, from a viewpoint of protection of environment, the use of chemical conversion treatment agent has difficulty in operability and a waste liquid produced after treatment. Accordingly, there has been a demand for a non-chromium surface treatment agent which can impart corrosion resistance and adhesiveness equal to or higher than corrosion resistance and adhesiveness brought about by the chromium-based surface treatment agent to the aluminum sheet.
With respect to the non-chromium surface treatment agent, for example, patent document 2 discloses an aluminum-use surface treatment agent containing zirconium and/or titanium, phosphate and a fluoric material.
Further, patent document 3 discloses a metal surface treatment agent containing a water-soluble zirconium compound, a water-soluble or water-dispersing acrylic resin, and a water-soluble or water-dispersing thermosetting cross-linking material. The content of the water-soluble zirconium compound is 500 to 15000 ppm on the basis of mass as zirconium, and the content of acrylic resin is 150 to 740 mgKOH/g in solid content acid value and 24 to 240 in solid content hydroxyl value and is 500 to 30000 ppm on the basis of mass as the solid content. The content of the thermosetting type crosslinking agent is 125 to 7500 ppm on the basis of mass as the solid content.
Further, in the chemical conversion treatment which contains an organic material in the chemical conversion compound disclosed in patent document 3, although adhesiveness between the aluminum substrate and the organic resin is enhanced, there may be case where a sludge derived from the organic material and aluminum ions are generated during the continuous treatment and this sludge gives rise to a drawback that the adhesiveness is deteriorated after coating organic resin or after formation of a formed body.
Further, in patent document 4, sorbitol is exemplified as an aluminum ion sequestering agent. A treatment liquid which contains the aluminum ion sequestering agent is an aqueous alkaline cleaning liquid before the chemical conversion treatment and is not a chemical conversion treatment liquid. Further, sorbitol is provided for sequestering aluminum ions generated by etching and hence, the technical concept disclosed in patent document 4 differs from the technical concept of the present invention.
[Patent document 1] JP-A-5-125555
[Patent document 2] JP-B-56-33468
[Patent document 3] JP-A-2002-275648
[Patent document 4] JP-A-9-111465